It is often desirable to seal a bottle, jar or other container with a closure to maintain freshness of the contents thereof or to indicate whether the container has been tampered with. However, it is also desirable that the closure be easy to remove by the user. For example, U.S. Pat. No. 5,433,992, the contents of which are incorporated herein by reference, describes a top-tabbed closure for a container which has a membrane for sealing the container and a sheet which is bonded to the top of the membrane, in a manner which leaves a tab portion of the sheet free. A user seeking to gain access to the contents of the container simply grips the tab with their fingers and by pulling on the tab, which is connected to the sheet, can remove the entire closure and access the contents of the container in a relatively convenient manner.
Referring generally to FIG. 1, a conventional top-tabbed closure is shown generally at the top of a bottle 10 as container seal 100. A cross sectional view of seal 100, taken along line 2-2 of FIG. 1 which is not drawn to scale, is shown in FIG. 2.
Seal 100 includes a lower section 101, comprising a lower layer 110, which is formed of an adhesive, such as a hot melt adhesive or other sealants, for securing seal 100 to the top of bottle 10. Lower section 101 also includes a foil layer 120 and a polyethylene terephthalate (PET) layer 130 between foil layer 120 and sealant 110. Seal 100 also includes an upper section 102. Upper section 102 includes an ethylene vinyl acetate (EVA) layer 170 having a PET top layer 180 disposed thereon. A bottom surface 150 of EVA layer 170 is surface treated and bonded to foil layer 120. Lower surface 150 also bonds a paper release layer 140 to EVA layer 170. Thus, release layer 140 prevents EVA layer 170 from being completely bonded to foil layer 120 at lower surface 150. Lower surface 150 only bonds EVA layer 170 to foil 120 up to a boundary line 160 so as to permit a tab portion 200 to be graspable. However, this bond between upper section 102 and lower section 101 is strong enough, so that pulling tab portion 200 can remove all of seal 100 in one piece.
Conventional container seals exhibit several problems. For example, a paper release or information layer can be sensitive to exposure to moisture. Use of PET release layers alone do not provide a fully satisfactory seal. Corrosion of foil layers can also present a problem. Also, conventional closures typically require containers to have smooth surfaces to insure proper bonding and release.
One particular problem is uneven heating during heat sealing steps. The uneven heating of the heat-activated adhesive is attributed to the uneven distribution of heat between the tabbed and non-tabbed sides of the seal. As a result, one side of the seal is sufficiently adhered to the mouth of the container while the other side is not. The common solution to this problem has been overheating (i.e., oversealing) the seal to ensure that both sides of the seal are adhered to the container. However, this common solution presents additional problems in that the closures will not separate from the container satisfactorily when the tab is pulled which can result in tearing and unsatisfactorily incomplete removal of the seal from the mouth of the container. Likewise, the use of excessive heat can lead to oozing of the sealing adhesive which in turn can adhere the tab to the seal. This unwanted side effect is sometimes referred in the art as “tab grab” which prevents the end user from having easy access to the tab for removal of the seal from the container.
The shortcomings of the conventional top-tabbed container seals described above are heightened when the seal is combined in a two-piece liner and seal assembly. A two-piece liner and seal assembly typically includes an upper portion of a liner, compressing agent, or pulp board that is bonded by a wax or other material to an upper surface of a lower seal portion. The wax holds the liner portion to the seal portion prior to induction sealing. Upon induction heating, the wax melts and is absorbed by the liner to separate the upper liner portion from the lower seal portion and the lower seal portion is heat sealed to a container rim. Upon a consumer opening a cap or other closure, the liner is retained in the cap while the seal remains bonded to the container rim.
When the conventional top-tabbed seal mentioned above is combined with a wax bonded liner forming a two-piece induction seal, the induction sealing window of operation is considerable smaller than a comparable, but non-tabbed wax bonded liner and seal combination. The problems with uneven heating caused by the partial, paper release layer tend to insulate the wax layer and impede wax absorption over at least a portion of the assembly above the release layer at lower induction settings. Thus, wax may be adequately absorbed on the non-tabbed side of the seal to permit liner separation, but not fully melted and not absorbed on the tabbed side of the seal. This tends to result, in some cases, of a liner that has not properly separated from the seal above the paper release layer. In other cases, if the seal includes a foam layer, the foam tends to trap heat below it which may also result in the wax not being adequately absorbed. A common field solution to these problems is an elevated level of induction heating or a longer induction dwell time, which tends to generate a surplus of heat energy in the assembly. However, elevated heating or longer dwell times has an adverse effect on the performance of the two-piece assembly. For instance, the additional heat further accentuates the performance shortcomings of the tearing, adhesive oozing, and tab grab mentioned above. Moreover, in a two-piece liner and tabbed seal assembly, elevated induction heating and dwell times can often melt the upper polymer layers of the lower seal portion resulting in permanent adhesion or back bonding of the lower tabbed seal portion to the upper liner portion.